In the radio communication system such as a cellular system, a method for obtaining a macro-diversity effect by effectively utilizing a plurality of base stations has been introduced. Hereinafter, each transmission point belonging to a base station will be referred to as a “cell,” and an explanation will be given by exemplifying LTE (Long Term Evolution), which is a next generation communication system that has been examined in 3GPP (3rd Generation Partnership Project), that is, an international standards organization of mobile communication. In LTE, by introducing a hard handover process that appropriately switches connecting cells, transmission quality is ensured for signals transmitted from a transmitting apparatus (a base station and a cell) to a receiving apparatus (terminal), in communication in a downlink direction from each cell to a terminal.
In LTE-advanced (hereinafter referred to as “LTE-A”) that is a communication system with further advanced LTE, introduction of a coordinated multipoint transmitting/receiving process (CoMP) has been examined in an attempt to positively utilize the micro-diversity so as to improve the data rate mainly at a cell edge. As specific systems to be used for CoMP, mainly two methods have been examined in LTE-A, for example, as shown in Non-Patent Literature 1.
As specific systems to be used for CoMP, mainly two methods have been examined in LTE-A, for example, as shown in Non-Patent Literature 1.
(1) Joint processing/transmission (JP): simultaneous transmission of data from a plurality of cells toward a terminal.
(2) Coordinated beam-forming/scheduling (CB): time-frequency resources and space resources (beams) are assigned coordinately among a plurality of cells so as to reduce interference.
In these methods, unlike a simple switching process of connected cells of the hard handover in LTE, a simultaneous transmitting process and a coordinated controlling process are applied by the use of a plurality of cells.
FIG. 1 shows one example of receiving operations in terminals in JP. In LTE, as the minimum unit for use in arranging signals, a sub-carrier is defined in the frequency domain, and an OFDM symbol is defined in the time domain. Hereinafter, this is referred to as “time-frequency resource.” Among a plurality of time-frequency resources possessed by itself, each cell multiplexes a control signal (PDCCH: Physical downlink control channel) and a data signal (PDSCH: Physical downlink shared channel) in the time domain, and transmits the resulting signal. More specifically, in LTE, a sub-frame is defined as the minimum unit for use in frequency scheduling and adaptive MCS controlling in the time domain. In the same manner, a resource block (hereinafter referred to as “RB”) is defined as the minimum unit for use in frequency scheduling and adaptive MCS controlling in the frequency domain. Among 14 OFDM symbols located within the sub-frame, a single or a plurality of successive OFDM symbols from the leading portion are used for transmitting a control signal, while the rest of symbols in the sub-frame are used for transmitting a data signal. The determination of the number of OFDM symbols for control signals to be set in this case is carried out by each cell, and the value of 1 to 3 in the number of OFDM symbols is taken. Moreover, a cell-specific reference signal (CRS) relating to a frequency shift amount that is different among cells is also transmitted therefrom. In JP, an operation for assigning the same RB to a plurality of cells that participate in simultaneous transmission is carried out. This is indicated as PDSCH for CoMP in the figure. Moreover, as in the case of the PDSCH for non-CoMP region in the figure, a RB from which the above-mentioned resources have been excluded can be desirably assigned to a terminal to which each cell itself is connected. Furthermore, each of terminals for use in CoMP operation in JP receives a control signal from a predetermined single cell, and is notified of pieces of information as to the presence or absence of a data signal assignment to itself in the corresponding sub-frame, or as to which RB has been assigned thereto. The cell for outputting these instructions is defined as “serving cell,” and the other cells that participate in simultaneous transmission are referred to as “other cell A and other cell B” in the figure.
In this simultaneous transmitting method, signals located in the PDSCH for CoMP region are simultaneously transmitted from three cells (serving cell, other cell A and other cell B) so that, even when a signal from a specific cell is subjected to serious degradation in quality due to fluctuations, such as phasing and shadowing, detection is available by the signals from the other cells, that is, a macro-diversity effect is obtained, thereby making it possible to stabilize the received quality.